Minimally invasive surgical interventions have gained steadily in importance for years and are indispensable for example for the treatment of stenoses. Recently, they have also been used increasingly in the implantation of artificial heart valves. Suitable delivery catheters are known in a very wide range of designs and are the subject of on-going further development. In recent years, the development has focused particular attention on catheters which not only permit the placement, but also the removal or a repositioning (when necessary) of cardiovascular implants.
Delivery catheters of this type consist fundamentally of a first, inner catheter shaft, at the distal end of which there is arranged the implant. The implant and the first, inner catheter shaft are surrounded by a second, outer catheter shaft. The distal region of the second outer catheter shaft, which surrounds the implant, is often referred to as an implant capsule, or occasionally as a catheter sleeve. Here, the implant capsule can consist of the same material as the second outer catheter shaft or of another material connected to the second outer catheter shaft. Here, the position reference “proximal” denotes a part of the delivery catheter disposed closer to the user, and “distal” accordingly denotes a part of the delivery catheter disposed further away from the user.
The implants (in particular the heart valve stent) are often made of a shape-memory material. In these cases, the implants are held in their compressed form by the catheter sleeve surrounding them as they are delivered to the site of implantation. Displacement of the catheter sleeve, for example proximally, gradually releases the implant as the sleeve is withdrawn and permits the implant to expand.
Recently, solutions have been proposed which were supposed to permit a return (resheathing or recapturing) of an already partially released implant into the delivery catheter, i.e. especially into the implant capsule. Such resheathing is of great advantage to surgeons because the resheathing permits corrections during the delivery process and thus helps the implantation process to be concluded with the best-possible results. Especially in cases in which the implant is released by retracting the catheter sleeve proximally, relatively high reaction forces or high local stress concentrations occur as the heart valve stent is returned into the implant capsule or, more specifically, as the distal catheter end is drawn back over the stent. These forces and stresses lead to complex problems. See, e.g. Yeung et al., U.S. Pat. No. 8,562,673, entitled Stented transcatheter prosthetic heart valve delivery system and method. The '673 patent uses a tubular shape memory capsule in the form of a cut tube with longitudinal splines/wires that provide reinforcement. The capsule includes a distal zone that transitions to a flared state and imparts a collapsing force onto a prosthesis when the capsule is moved back over the prosthesis, causing the prosthesis to radially collapse and become recaptured within the delivery capsule. This process places high local stresses on a heart valve prosthesis at the point where the collapsing force is applied by the transition region.